The present invention relates to a solid-state image sensor and, more particularly, to a contact-type image sensor having a cell array of amorphous semiconductor photoelectric converting elements, which are switch-driven using a matrix drive technique.
Amorphous semiconductor image sensors are preferably used for various types of optical image reading equipment such as a facsimile system, an optical code reader, a copying machine, etc., since the use of such image sensors makes it possible to miniaturize the overall size of the equipment. In particular, considerable effort has been given to the development of contact-type solid-state linear image sensors having substantially the same length as the width of documents to be read. Such image sensors offer an advantage over other kinds of imaging devices in that they can eliminate image reduction that uses a lens system before the document image reaches thereto, so that the optical image reading equipment can be made compact in size.
The contact-type amorphous semiconductor image sensor is usually connected to a matrix wiring circuit and switch-driven by a matrix drive technique. In this case, linearly aligned photoelectric converting elements serving as pixels (picture elements) are divided into pixel groups or cell units, which are connected at their first comb-shaped planar electrodes (serving as individual cell electrodes) to an image signal detector, through a matrix wiring circuit consisting of crossed row and column signal line. In each pixel group a second comb-shaped planar electrode is provided to serve as a common electrode therefor. The second comb-shaped planar electrodes of the pixels are connected to a drive voltage generator. The photoelectric converting elements are successively selected with each pixel group as a unit, so that time sequential video signals may be obtained by the image signal detector.
According to the conventional image sensor constructed as described above, however, pixel signal current cannot be effectively prevented from leaking to neighboring signal lines through stray capacitance and/or leak resistance, which is inherently generated between planar electrodes or between the row and column lines of the matrix circuit. When current flowing in a certain signal line is partially leaked to the neighboring signal line, through which another pixel current is to be supplied, the leaked current will serve as a noise current for the image signal obtained from the selected photoelectric converter element. As a result, the signal-to-noise ratio of the image signal is decreased to degrade not only the picture quality of a reproduced video image but also the speed of the image reading operation. This problem remains as a serious bar to the development of compact, high-performance image reading equipment.